Observations of a Radial Density Gradient in the Very Local Interstellar Medium by Voyager 2

Beginning on 19 June (Day 171) of 2020, at a heliocentric distance of 124.2 AU, the Voyager 2 Plasma Wave Science (PWS) instrument began to observe radio emissions followed by electron plasma oscillations in its 3.11 kHz spectrum analyzer channel. Plasma oscillations at this frequency imply an electron density in the range of 0.12 cm^-3 ± 15%, although some response in the 1.78 kHz channel near the peak of the plasma oscillations suggest a density of 0.87 cm^-3 ± 8% for that time. Shortly after Voyager 2 crossed the heliopause, in late January 2019, the Voyager 2 PWS detected plasma oscillations in its 1.78 kHz channel giving an electron density of 0.039 cm^-3 ± 15%. While the Voyager spectrum analyzer affords relatively poor spectral resolution, the recent observation of plasma oscillations in the 3.11 kHz channel provides definitive evidence of a radial density gradient in the very local interstellar medium (VLISM), just beyond the heliopause with a magnitude similar to that observed by Voyager 1 obtained with higher spectral resolution measurements. Plasma oscillations observed by Voyager 1 range from frequencies as low as 2.1 kHz increasing to about 3.2 kHz, giving an electron density profile that increases from about 0.055 to about 0.13 cm^-3 over a distance spanning some 20 AU. Given the 67° difference in heliographic latitude and 43° difference in longitude between the two Voyagers, the new Voyager 2 observations imply the density gradient is a large-scale feature of the LISM, at least in the general direction of the heliospheric nose. A number of theories have been suggested to explain such a gradient but, as yet, there is no consensus. The plasma oscillations are the source of radio emissions in the frequency range of 1.8 to 3.6 kHz observed as early as 1983 when the two Voyagers were beyond about 12 AU where the solar wind plasma density is sufficiently low to allow such waves to propagate.